When timer operating water heating the central heating is coming on aswell?

In some conventional heating systems there's a valve that directs the heated water to either the radiators or to the hot water storage tank - or to both at the same time. You can usually find it in the airing cupboard - a box with wires - one pipe coming in and two pipes going out.

Sometimes these valves get stuck in the mid position and heated water goes to both places - you can take the motorised head off and operate the valve by hand to free it up.

Or call in a heating engineer.

Good Luck...
May 2016